The present invention relates to an endothermic furnace providing a protective atmosphere for heating metals. More particularly, the instant invention relates to an improved furnace construction for sintering metal using an indirect radiant tube heater both as the source of heat and as the original source for or as the replenishing source of the endothermic atmosphere during treatment of the metal.
A metal heating furnace is characterized by the presence of a closed work area in which the work is heated to the temperature necessary to perform the metallurgical process. Heat can be achieved by using electric heating elements or the firing of indirect radiant tube heaters. If fired radiant tubes are used as the source of heat, the combustion products are conventionally vented to the atmosphere. In such processes, an atmosphere of very specific and controlled composition, taken from a source external to the furnace, is introduced into the heated work area in order to protect the work from undesirable metallurgical reaction such as oxidation, or to impart specific metallurgical characteristics to the work, for example carburizing or carbonitriding. This is very costly and an arrangement which recovers the combustion products for use as the furnace atmosphere would be desirable.
One object of the present invention is to take advantage of the combustion products present in the retort tubes and to utilize the combustion products by passing them through a catalyst in the retort to create an endothermic atmosphere which is used in the protection of the metals in the high temperature portion of the sintering furnace. The treated combustion products are used either as the carrier gas or as the protective atmosphere for the metallurgical process that is carried out in the furnace.
In accordance with this invention, furnace atmosphere gas is generated by the action of a catalyst which is located in each of a pair of elongated U-shaped retort tubes, each retort tube being supported from an elongated rectangular-shaped lid removably secured to the high heat chamber of the furnace and the high heat chamber of the furnace being externally heated to approximately 1900.degree.-2200.degree. F. (1040.degree.-1200.degree. C.). Air and gas external to the chamber are mixed, pressurized and then passed through the catalyst whereupon the mixture is combusted. An endothermic heat-absorbing reaction works on the heated combustion products to produce the desired furnace atmosphere gas. The completely reacted furnace atmosphere gas is rapidly cooled in a first cooler assembly to prevent the formation of soot. An array of finned selectively interconnected heat exchanger tubes define the cooler assembly with each retort tube having an outlet end thereof associated with an inlet to one heat exchanger tube to pass the gas through the related heat exchanger tubes. Thereafter the cooled gas is discharged from an outlet from another heat exchanger tube and returned to the work area of the furnace to provide a predetermined atmosphere for the metallurgical process.
In such process it is important that the furnace atmosphere not be contaminated and thus the work area of the furnace must be airtight. Should one of the U-shaped retort tubes crack during operation, removability of the lid allows for replacement of cracked tubes and replacement of the catalyst.
Positioning of the retort tubes on the lid such that the long dimension of each tube is parallel to the long dimension of the lid maximizes the path of the combusted air and gas mixture in the high heat chamber, thereby increasing the exposure of combusted gas products to the catalyst, maintaining a constant flow of combusted gases through the retort tubes, and enhancing the uniformity of heat radiated from the retort tubes by the combusted gas products. Increasing the length of the retort tube advantageously increases the amount of catalyst which undergoes an endothermic reaction with combusted gas products thereby generating an excess supply of oxygen-free gas. Increasing the amount of gas which is reacted can possibly generate a supply of furnace atmosphere to provide two furnaces with a controlled furnace atmosphere.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken into conjunction with the accompanying drawings: